You are probably right...... This is how it starts most often. You haven't said what O/S but that will not make a difference, except perhaps to what can be, or is worth trying to recover from, the old HD.
You didn't say if you had tried booting from a floppy or a CD with the idea of running virus check, though that is not very high on my list of likely causes
Sounds like time to fit a new Primary HD, get everything up and running and then see what can be saved from the old 'un now set up as a Secondary HD.